In a tubular refractory member having an inner bore for allowing molten steel or other high-temperature substance to stay therein or pass therethrough, such as a long nozzle for discharging molten steel from a ladle into a tundish, or an immersion nozzle for pouring molten steel from a tundish into a continuous casting mold (these nozzles will hereinafter be referred to collectively as “continuous casting nozzle”), a temperature gradient occurs between an inner bore-side region and an outer periphery-side region of the refractory member. Particularly, in an initial stage of discharging/passing of molten steel, the inner bore-side region or the outer periphery-side region is rapidly heated up, so that the phenomenon becomes prominent.
Irrespective of whether the refractory member has a single-layer structure or a multi-layer structure, such a temperature gradient gives rise to a strain due to an internal stress of the refractory member, which becomes one factor causing breaking, such as cracking in the outer periphery-side region. Further, as the temperature gradient becomes larger, and a thermal expansion coefficient of the inner bore-side region becomes larger with respect to that of the outer periphery-side layer, a thermal stress will be increased to cause a higher risk of breaking, particularly, in the outer periphery-side region.
As commonly-used measures against breaking due to the temperature gradient (thermal stress), there have been known various thermal-stress reduction techniques based on an increase in thermal conductivity, a reduction in thermal expansibility, a reduction in elastic modulus, etc., such as a technique of incorporating a large amount of graphite into a refractory material for a continuous casting nozzle, and a technique of adding fused silica with a small thermal expansion amount to a refractory material for a continuous casting nozzle or increasing a content of fused silica in a refractory material for a continuous casting nozzle. However, on the other hand, the increase in amount of graphite or fused silica involves deterioration in oxidation resistance and increase in reactivity with components of molten steel. This has a disadvantage of giving rise to deterioration in durability, such as erosion (abrasion) resistance and corrosion resistance, particularly, of the inner bore-side region.
A continuous casting nozzle is used under a condition that a molten steel flow passes through an inner bore thereof, while violently colliding against an inner bore surface thereof Thus, a region of the continuous casting nozzle adjacent to the inner bore surface will be particularly severely damaged due to abrasion (erosion) caused by the molten steel, non-metal inclusions in the molten steel, etc., structural embrittlement and washing (corrosion) caused by oxidizing components of the molten steel, etc., and melting loss caused by a reaction product with FeO and other components of the molten steel.
Further, recent years, in connection with an increase in amount of non-metal inclusions (such as alumina) in molten steel, attachment of inclusions (typically, alumina) onto the inner bore surface of the continuous casting nozzle, or clogging of the inner bore of the continuous casting nozzle due to the inclusions, become one key factor determining a lifetime of the continuous casting nozzle.
In the above circumstances, there has been an increasing need for a higher level of durability and safety (stable casting capability) of the continuous casting nozzle.
With a view to meeting the above need, it has been attempted to extend a lifetime of a continuous casting nozzle, in such a manner that a refractory material excellent in thermal shock resistance is used for a nozzle body (i.e., an outer periphery-side layer) of the continuous casting nozzle to form a backbone portion of the continuous casting nozzle, and a refractory material excellent in durability, such as erosion resistance and corrosion resistance, is disposed as an inner bore-side layer defining an inner bore surface for contact with molten steel.
In particular, with regard to the inner bore-side layer, various efforts for functional enhancement have been carried out. Therefore, lately, it has not been uncommon to define the inner bore surface by a lining made of a material reduced in content of carbon, a graphite-free material, or a material containing a component excellent in erosion resistance, melting-loss resistance, etc., such as a basic component. Further, with a view to reducing or preventing attachment of inclusions (such as an alumina component) onto an inner bore surface of an immersion nozzle, or clogging of an inner bore of the immersion nozzle due to the inclusions, one type of immersion nozzle has been increasingly used in which a refractory layer containing a CaO component highly reactive with an alumina component is provided on an inner peripheral surface of a nozzle body thereof.
The highly-functional refractory material contains a small amount of a material having a high capability to relax thermal expansion, such as graphite, and a large amount of refractory aggregate having high thermal expansibility. Thus, a thermal expansion amount of the inner bore-side layer is apt to be increased. Moreover, due to an increase in thermal gradient caused by an increase in thermal conductivity of the inner bore-side layer with respect to the outer periphery-side layer as a result of the reduction in carbon content, a difference between respective thermal expansion amounts of the inner bore-side layer and the outer periphery-side layer, and a resulting thermal stress, are apt to be more increased, which leads to a higher risk of breaking of the continuous casting nozzle, particularly, the outer periphery-side layer.
As an example of an approach to preventing the breaking due to a thermal stress of a highly-expansible inner bore-side layer, the following Patent Document 1 discloses a continuous casting nozzle which comprises a refractory sleeve prepared to contain CaO in an amount of 20 mass % or more and inserted into a nozzle body thereof, wherein a bonding material comprising a mixture of a refractory aggregate and a binder is applied onto a part or entirety of an outer peripheral surface of the sleeve or an inner peripheral surface of the nozzle body, or into a joint region defined between the outer peripheral surface of the sleeve in an inserted state and the inner peripheral surface of the nozzle body, and wherein the bonding material for the joint region is adjusted to have a porosity of 15 to 90% in a dried state thereof The Patent Document 1 discloses that the porosity of the bonding material for the joint region is adjusted by means of an increase/decrease in amount of the binder and a solvent each constituting the bonding material or a change in filling amount of the bonding material. This technique is intended to obtain a stress relaxation capability based on the porosity of mortar, i.e., voids in a mortar structure, and adjust a level of stress relaxation capability by means of an increase/decrease in amount of the binder and the solvent each constituting the mortar (bonding material) or a change in filling amount of the mortar.
However, in the above adjustment technique, a large amount of liquid (solvent and binder) is required to obtain a high stress relaxation capability, so that the mortar is liable to have fluidity. For example, this has a disadvantage of significant deterioration in shape retainability of the mortar to cause difficulty in ensuring a required thickness of a mortar layer or a fully filled state of the joint region. More specifically, in an operation of installing an inner bore-side layer in a nozzle body (i.e., an outer periphery-side layer) of a continuous casting nozzle, using such mortar having a high fluidity or low shape retainability, it is highly likely that the inner bore-side layer is displaced to cause an undesirable situation where the mortar layer has a region having almost no thickness, a region having an excessively large thickness, and/or a large number of void spaces. This situation precludes a possibility to ensure required capabilities, such as the stress relaxation capability and a capability of preventing molten steel and other foreign substances from intruding into the joint region, which leads, particularly, to breaking of the outer periphery-side layer and drop-off of the inner bore-side layer.
The above mortar layer is inevitably formed in a low-density structure and a weak binding structure, and thereby a structural strength thereof becomes lower. Thus, even if the inner bore-side layer can be fixed to the outer periphery-side layer (nozzle body of the continuous casting nozzle) in an intended relative arrangement through the mortar layer, the mortar layer is likely to be broken not only by an expansion force during stress relaxation under a hot condition but also by a weak external force during handling of the nozzle, to cause difficulty in maintaining structural stability. This gives rise to a problem that peeling, displacement or the like of the inner bore-side layer is likely to occur.
The above mortar layer has a high porosity, wherein a large continuous pore exists in the mortar structure. This gives rise to another problem that molten steel, slag component and other foreign substances are infiltrated in the mortar layer through the pore (including a broken and enlarged pore) as a pathway to cause melting loss or breaking of the mortal layer itself.
The above mortar contains a large amount of liquid to ensure adequate working efficiency during mortaring. Thus, the liquid is liable to be absorbed in target refractory layers to be bonded, to cause a change in concentration of a solid content of the mortar. This means that, if each of adjacent refractory materials has a different apparent porosity, a solvent contained in the mortar to provide flexibility and bindability thereto is absorbed in the refractory materials through contact therewith, and thereby compressibility and bonding strength of the mortar are changed in each region, which gives rise to a problem that the compressibility and bonding capability become instable depending on adjacent refractory materials and a thickness of a mortar joint. Further, during a course of the absorption and drying, the liquid is liable to cause a problem that shrinkage or crack occurs in the mortar layer itself, or gap or peeling occurs between the molar layer and each of the target refractory layers. Moreover, along with a reduction of amount of the liquid in the mortar, aggregate particles will agglomerate together, which is likely to give rise to a problem concerning bonding capability due to a higher risk of the occurrence of crack, peeling or the like in the mortar layer.
Further, the following Patent Document 2 discloses a casting nozzle having a two-layer structure in which only a portion of the nozzle on the side of an inner bore thereof (inner bore-side layer) is formed as a carbon-free refractory layer, i.e., a refractory layer having high thermal expansibility and high corrosion resistance, and the remaining portion on the side of an outer periphery thereof (outer periphery-side layer) is formed as a carbon-containing refractory layer, i.e., a refractory layer excellent in spalling resistance, wherein at least 80% or more of a contact surface between the inner bore-side and outer periphery-side layers is separated from each other by a separation layer which is formed by setting a burnable material, such as polypropylene or nylon, between the two layers, and then burning away the burnable material, during forming/shaping of the nozzle.
However, in the casting nozzle disclosed in the Patent Document 2, less than 20% of the contact surface between the inner bore-side and outer periphery-side layers is bonded together. Even if a bonded region is fairly small, it will be an origin of a phenomenon that splitting occurs in the outer periphery-side layer due to thermal expansion of the inner bore-side layer (hereinafter referred to as “expansion splitting”), because a stress causing the expansion splitting is transmitted from the inner bore-side layer to the outer periphery-side layer through the bonded region. If the bonded region is set at zero %, it causes a fundamental problem that the inner bore-side layer cannot be structurally supported. Moreover, in the separation layer in the Patent Document 2, i.e., a so-called hollow joint, molten steel easily intrudes into a void space of the joint, which gives rise to problems, such as cracks in the refractory layers due to solidification shrinkage of the molten steel occurring when it undergoes changes in temperature and expansion of the solidified steel occurring when it is heated, and peel-off of the inner bore-side layer due to no bonding between the inner bore-side and outer periphery-side layers.
Particularly, in a continuous casting nozzle comprising an inner bore-side layer, an intermediate layer and an outer periphery-side layer, where an MgO—CaO based material is used for the inner bore-side layer, depending on respective compositions of the inner bore-side layer and the intermediate layer, a damage, such as melting/washing, is rather likely to occur beyond a bonded region where the inner bore-side layer is in direct contact with the intermediate layer, which causes problems, such as melting loss, peel-off or reduction in fixing strength of the inner bore-side layer, breakup of the intermediate layer, formation of a hollow space between the inner bore-side and outer periphery-side layers, and intrusion of molten steel into the hollow space.
As above, in a continuous casting nozzle having a highly-expansible inner bore-side layer provided inside an outer periphery-side layer, a stress relaxation layer is required to have a capability to relax a stress to be caused by thermal expansion of the inner bore-side layer, a shape retainability allowing a required thickness and a filled structure to be obtained during an installation operation without a large continuous pore causing intrusion of molten steel and slag components, a structural strength enough to avoid breaking by an external force which is less than a stress caused by thermal expansion of the inner bore-side layer, and a supportability enough to prevent the inner bore-side layer from being peeled off from the outer periphery-side layer. However, any mortar layer having all the capabilities has not yet been obtained.                [Patent Document 1] Pamphlet of International Publication No. 03/086684        [Patent Document 2] JP 7-232249A        